This invention pertains to a method for determining silicon (mass 28) beam purity prior to implanting gallium arsenide. The most common implant dopant in gallium arsenide is silicon. Silicon has three isotopes, mass 28, is mass 28. An ion implanter uses magnetic mass analysis to choose the implant species. However, there are two common impurities, in an implanter that also have mass 28. These other impurities which the implanter cannot distinguish from mass 28 silicon, are N.sub. 2 and CO. The amounts of N.sub. 2 or CO in the ion beam that passes through the magnet will depend on many variables such as the vacuum in the source area, when the source was last cleaned, how long since the source was vented and the number of carbon apertures that are in the vicinity of the ion source. These variations in percent of carbon monoxide or nitrogen in the silicon beam can cause variation in dose and resultant carrier activation.
In prior art the implantation of carbon monoxide or nitrogen is avoided by implanting silicon mass 29 or mass 30. However, the natural abundance of these species is less than 5% in each case, causing the beam current to be quite small and the time required to implant a wafer to be accurate dose integration becomes difficult.
Unlike implants in silicon where a monitor wafer can be implanted, annealed and measured quickly to see if the implanter is working properly, gallium arsenide must be capped prior to anneal and the cap must be removed subsequent to the anneal in order to probe, greatly increasing the turn aroundtime. Also the use of gallium arsenide wafers as monitors is much more expensive as compared to silicon wafers. Further, in the prior art Si wafers could not be used as monitors since Si implants in Si would not be electrically active.